This invention relates to a drive belt tension control apparatus for a powered machine such as a power tool of the type wherein tension on one or more rotating parts is provided by pneumatic pressure through the operation of a pneumatic piston and cylinder assembly. The particular embodiment of a power tool incorporating the principles of the invention and disclosed in this application is an inverted router.
An inverted router is a particular type of woodworking machine wherein the operator is protected from the router bit by positioning the workpiece on a table and making cut in the workpiece from the underside. The table has on opening through which the router bit protrudes. The router bit is mounted on a spindle. A foot-operated pedal controls movement of the router bit upwardly through the table opening into workpiece engagement and downwardly out of workpiece engagement. The router bit moves vertically on a precise axis in order that the cuts be precisely positioned. This axis is defined by a guide pin which is positioned on the same axis and is used to guide the workpiece on the table.
In most types of inverted routers the spindle is driver by a motor through a pulley and belt arrangement. It is important to maintain the tension on this belt within a relatively narrow range. If the belt tension is too high the spindle may be deflected off of the proper rotational axis and the life of the bearings in the spindle assembly substantially shortened. If the tension is too low, slippage between the belt and one or both of the pulleys can reduce the speed of the spindle to below its optimum speed, especially when the spindle is under load while a cut is being made. Reduced spindle speed can also cause the router bit to chatter in the workpiece, stop, and/or cause excessive wear to the belt and pulley.
Prior art belt tensioning devices comprise brackets which are loosened while the tension is adjusted, and then tightened to maintain the tension at that point. However, this method is highly inaccurate and time consuming. Even if the tension is adjusted properly, belt elongation, temperature, humidity and vibration can very quickly cause the tension to move out of proper adjustment. Ideally, tension should be controlled constantly, and should be variable, if necessary, to take into account environmental and operating conditions. Using the motor as a tensioning weight cannot be done since the motor and spindle must be maintained in proper axial alignment with each other.
A workable and efficient solution to this problem was developed by applicant and is disclosed in U.S. Pat. No. 4,893,661. In the '661 Patent, the motor pulley of the router is mounted for continuously variable movement, either on a slide arrangement, or by pivoting the motor pulley on a vertical axis. Then a predetermined pressure is applied to the motor pulley which tensions the drive belt connecting the motor pulley and the spindle carrying the router bit. In the '661 Patent, the pressure is applied by a pneumatic piston and cylinder assembly. Thus, proper tension is maintained at all times. Tension is easily adjusted by varying the air pressure to the piston and cylinder assembly.
This solution provides excellent tension control. However, it has been observed that under certain conditions the tension on the spindle is sufficiently great that when the spindle is moved upwardly into cutting position, a certain amount of hesitation is experienced. This is the result of having to reach a certain level of force to overcome vertically stationary inertia of the spindle and the force with which it is being held in its vertical position by the tension on the drive belt. The invention of this application solves this problem without any detrimental effect on the precise tension control achieved through the use of the tension control apparatus disclosed in the '661 Patent.